Implement mad_hi built-in

We already have a working mul_hi, and the spec gives us the implementation as:
Returns mul_hi(a,b)+c.

Signed-off-by: Aaron Watry <awatry@gmail.com>
Reviewed-by: Tom Stellard <thomas.stellard@amd.com>

git-svn-id: https://llvm.org/svn/llvm-project/libclc/trunk@190211 91177308-0d34-0410-b5e6-96231b3b80d8